USING DISCONTINUITY SURFACES FOR STRATIGRAPHIC DISCRIMINATION IN THE UPPER ORDOVICIAN LEXINGTON LIMESTONE, CENTRAL KENTUCKY, U.S.A

Although previously thought to be an example of “layer-cake” stratigraphy, U.S.G.S. mapping of the Upper Ordovician Lexington Limestone of central Kentucky has shown that the unit is mostly a facies mosaic developed on a structurally controlled carbonate buildup. Up to 16 different members have been described in the unit, but understanding the relationships between members on different parts of the buildup has been difficult because of the rapid and abrupt stratigraphic changes throughout and the apparent lack of time-equivalent surfaces for correlation. However, the Lexington Limestone is characterized by hundreds of discontinuity surfaces with iron-stained hardgrounds, and in an effort to understand stratigraphic relationships, we examined these surfaces in order to see if any of them represented isochronous breaks in sedimentation that could be used for correlation. Although most of the surfaces are not correlative beyond the exposure where they occur, we did find three major surfaces that have correlative value based on stratigraphic continuity, degree of erosion and corrosion, and other physical characteristics like hardground thickness. These surfaces, the Sub-Brannon, sub-Sulphur Well and sub-Millersburg discontinuities, can be traced throughout central Kentucky and represent widespread interruptions in sedimentation with no structural discordance; hence, they are ideal for local correlation and their validity has been confirmed by isotope stratigraphy throughout the area. In fact, the three surfaces are interpreted to represent fourth- or fifth-order sequence boundaries or maximum-flooding surfaces that reflect regional tectonic activity and/or eustatic sea-level changes. The use of these correlative surfaces along with isotope stratigraphy in nine cross sections throughout the area has enabled us for the first time to clearly discriminate stratigraphic/facies relationships and to establish approximately isochronous facies maps at various time intervals throughout the Lexington Limestone. Although identifying discontinuity surfaces may be problematic, where such surfaces can be distinguished, they may have utility for correlation in other complex units like the Lexington Limestone.